Missing Cannell child, Philadelphia PA before 1900

I found my grandparents, Humberston Skipwith Cannell and Susan Ridgeway Miller Cannell, in the 1900 census in Philadephia, Pa with my father, Humberston Skipwith Cannell, Jr. and his older sister, Sarah Skipwith Cannell. In answer to the question: "Mother of how many children?", my grandmother answered three (3) and to the question: "How many living?", she answered two (2). I haven't been able to find any information on the birth or death of another child. Can you help me find this missing child? Mary